1. Field of the Invention
The present invention relates generally to wireless communications, and more particularly, to a method and apparatus for resource mapping for ACK (ACKnowledgment) and NACK (Negative ACKnowledgement) in a PDSCH (Physical Downlink Shared Channel) in a traffic adaptive TDD (Time Division Duplexing) system, when TDD Uplink-Downlink (UL/DL) configuration of a cell dynamically changes with changes in uplink and downlink traffic loads.
2. Description of the Related Art
LTE (Long Term Evolution) supports two duplexing modes, i.e. FDD (Frequency Division Duplexing) and TDD. FIG. 1 illustrates a frame structure in a conventional LTE TDD system. Each radio frame has a length of 10 ms, and is divided uniformly into two half-frames each of which having a length of 5 ms. Each half-frame includes 8 time slots of 0.5 ms and 3 special fields of 1 ms, i.e. DwPTS (Downlink Pilot Time Slot), GP (Guard Period) and UpPTS (Uplink Pilot Time Slot). Each sub-frame is composed of two consecutive time slots.
Transmission in a TDD system includes: transmission from a base station to a UE (User Equipment) (referred to as downlink transmission) and transmission from a UE to a base station (referred to as uplink transmission). According to the frame structure shown in FIG. 1, there are 10 sub-frames shared by uplink and downlink in every 10 ms. Each sub-frame may be allocated to uplink or downlink, and a sub-frame allocated to uplink is referred to as an uplink sub-frame, a sub-frame allocated to downlink referred to as a downlink sub-frame. TDD systems support 7 types of uplink-downlink configurations as shown in Table 1, where D represents Downlink sub-frame, U represents Uplink sub-frame, and S represents Special sub-frames in the 3 special fields.
TABLE 1Con-figurationserialSwitch-pointsub-frame indexnumberperiodicity01234567890 5 msDSUUUDSUUU1 5 msDSUUDDSUUD2 5 msDSUDDDSUDD310 msDSUUUDDDDD410 msDSUUDDDDDD510 msDSUDDDDDDD610 msDSUUUDSUUD
An advanced version of the LTE technique has been proposed to increase a transmission data rate of users. The advanced version of the LTE TDD system has the same HARQ (Hybrid Automatic Repeat Request) transmission timing with LTE. The following is a brief introduction to HARQ transmission timing of downlink data in LTE and the advanced version of LTE.
HARQ-ACK of PDSCH may be transmitted in a PUSCH (Physical Uplink Shared Channel) or a PUCCH (Physical Uplink Control Channel). Regarding the PDSCH to PUCCH/PUSCH timing, supposing a UE feeds back HARQ-ACK in PUCCH/PUSCH in sub-frame n, the PUCCH/PUSCH indicates HARQ-ACK information of PDSCH in downlink sub-frames n−k or HARQ-ACK information on SPS (Semi-Persistent Scheduling) release.
Table 2 provides definition of the value of kϵK. K is a collection of M elements {k0, k1, . . . , kM−1}, is related with sub-frame serial number and UL/DL configurations, and is referred to as a downlink association set. Element k is referred to as downlink association element. Herein, downlink sub-frames corresponding to a downlink association set are referred to as bundling window for short, i.e. for all elements k in K, a collecting composed of n−k is referred to as a bundling window, {n−k, kϵK}. Apiece of PUCCH resource in a PUCCH sub-frame is allocated for each PDSCH in each downlink sub-frame for feeding back HARQ.
TABLE 2Configurationsub-frame nserial number01234567890——6—4——6—41——7, 64———7, 64—2——8, 7,————8,——4, 67,4, 63——7, 6, 116, 55,—————44——12, 8,6, 5,——————7, 114, 75——13, 12,———————9, 8, 7,5, 4, 11,66——775——77—
With demands for a larger transmission data rate, a traffic adaptive TDD technique has been discussed in terms of higher LTE versions. The traffic adaptive TDD technique dynamically adjusts the ratio of uplink sub-frames to downlink sub-frames to keep the current uplink-downlink configuration more consistent with the ratio of uplink traffic load to downlink traffic load, to increase uplink-downlink peak rate of users and system throughput.
In a traffic adaptive TDD system, the UL/DL configuration adopted for timing from PDSCH to PUCCH/PUSCH may be different from the actual configuration adopted in the system, e.g. a UL/DL configuration may be specified via upper layer signaling, and HARQ-ACK of PDSCH is fed back according to the timing corresponding to the UL/DL configuration regardless of the actual UL/DL configuration adopted by the system. In order to have HARQ-ACK of PDSCH in more sub-frames be fed back, the configuration specified via upper layer signaling usually includes a relatively larger number of sub-frames. For example, the TDD UL/DL configuration specified may be UL/DL configuration 2 while the actually adopted TDD UL/DL configuration may be configuration 0, configuration 1, or configuration 6. Since downlink sub-frames covered by the actually adopted configuration is a sub set of the downlink sub-frames covered by the specified configuration, and the resource of HARQ-ACK fed back for PDSCH in all of the downlink sub-frames covered by the specified configuration can be found in uplink sub-frames, the resource of HARQ-ACK fed back for PDSCH in all of the downlink sub-frames covered by the actually adopted configuration can be found in uplink sub-frames. Thus, HARQ-ACK of all downlink sub-frames covered by the actually adopted configuration can be fed back as long as the downlink sub-frames covered by the actually adopted configuration is a sub set of the downlink sub-frames covered by the specified configuration.
In practice, higher version UEs and lower version UEs co-exist. The higher version UE refers to a UE using a higher version of LTE standards, and the lower version UE refers to a UE using a lower version of LTE standards. For example, a higher version UE may support dynamic traffic adaptive TDD while a lower version UE may not support dynamic traffic adaptive TDD. For PDSCH to PUCCH/PUSCH timing, a lower version UE and the higher version UE may adopt different UL/DL configurations. Due to the different configurations adopted, one uplink sub-frame may adopt different bundling windows, which is the size of downlink subframes fed back in this uplink subframe.
FIG. 2 illustrates a higher version UE configuration and a lower version UE configuration, according to the prior art. In FIG. 2, D and S in a sub-frame represent downlink sub-frames, and U represents uplink sub-frames. A higher version UE adopts the timing scheme of UL/DL configuration 2 for feeding back HARQ-ACK of PDSCH, and a lower version UE adopts the timing scheme of UL/DL configuration 0 for feeding back HARQ-ACK of PDSCH. When feeding back HARQ-ACK of PDSCH in the same uplink sub-frame 201 according to respective timing schemes, the higher version UE feeds back HARQ-ACK for downlink sub-frames in dashed box 202, and the lower version UE feeds back HARQ-ACK for downlink sub-frame 203. The dashed box 202 denotes a bundling window. Sub-frames 206 and 203 have the same serial number. HARQ-ACK of sub-frame 206 may be fed back in PUCCH resource field of the lower version UE, while HARQ-ACK of other downlink sub-frames 204, 205 and 207 may be fed back in other PUCCH resource fields.
In addition, the configuration specified by upper layer for a higher version UE may cover more downlink sub-frames. A conventional manner may reserve PUCCH resources for each downlink sub-frame covered by the specified configuration and decrease the downlink sub-frames covered by the actually adopted configuration to be fewer than those covered by the specified configuration. Thus, not all of the reserved PUCCH resources are used, resulting in a waste of PUCCH resources.
Accordingly, there is a need for solutions to the problem regarding compatibility between higher version UEs and lower version UEs and to the problem of wasting PUCCH resources during transmission of HARQ-ACK of PDSCH.